1. Field of the Invention
This invention relates to ultrasound imaging systems.
2. Description of the Prior Art
Ultrasound imaging systems are attractive for medical applications because they permit imaging of internal structures of the body without the use of potentially harmful forms of radiation. Numerous proposals have been made for such systems and many are in actual use. Although the systems vary widely in detail, most known systems basically utilize an ultrasonic transducer or array of transducers having coplanar emitting surface structure to beam pulses of ultrasound into a structure to be imaged and to receive reflections of those pulses. The beamed pulses are directed through the structure to be imaged in a suitable scanning pattern by mechanical and/or electronic means. One such form of scanning is known as "B-scanning".
An example of a known type of B-scanning ultrasonic system is disclosed in U.S. Pat. No. 4,014,207, issued on Mar. 29, 1977, to Meyer, et. al. for SECTOR SCANNING ULTRASONIC INSPECTION APPARATUS, which is herein expressly incorporated by reference.
A problem with such systems is lack of optimum resolution in the lateral direction, i.e., along the plane of the transducer emitting surfaces. Some improvement can be obtained utilizing large aperture transducers or transducer arrays which are electronically or acoustically focussed and/or scanned but this improvement is at the expense of a very shallow depth of field. Consequently, sophisticated means have been employed to maintain ultrasonic energy dynamically focused to a succession of points throughout a scan. The operation of such complex means, however, tends to limit the rate of scan and thus increase the time taken to generate an image. Moreover, even with such means, resolution is sometimes inadequate for reliable detection of small anomalies in the subject structure being imaged.
An example of a dynamically focused ultrasonic B-scanner is described in U.S. Pat. No. 3,090,030, issued on May 14, 1963 to Schuck for VARIABLE FOCUS TRANSDUCER. Another variant of a focused ultrasonic system is described in the following pulbication: Melen, R. D., et. al., "CCD Dynamically Focused Lenses for Ultrasonic Imaging Systems",
Proceedings of the International Conference on Applications of CCD's, 1975, pp. 165-171.